Hillary won't quit because she is infused with power-lust and an obdurate unwillingness to acknowledge (to herself) that she has any failings. She is like her husband in both respects.
Hillary, given her psychological inability to accept defeat in any form, is now grasping for the life-raft of racism. She believes that she is electable because she is white, whereas Barack Obama is unelectable because he is black. From that belief springs a further one: Enough superdelegates will perceive the "truth" of her belief before it is too late, and they will swing the nomination to her.
P.S. WSJ.com's James Taranto ("Best of the Web Today") offers a similar analysis of Hillary's "white strategy," an analysis that I hadn't read at the time of posting.